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Polynomial Structures for Nilpotent Groups

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Polynomial Structures for Nilpotent Groups TRANSACTIONS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 348, Number 1, January 1996 POLYNOMIAL STRUCTURES FOR NILPOTENT GROUPS KAREL DEKIMPE, PAUL IGODT, AND KYUNG BAI LEE Abstract. If a polycyclic-by-finite rank-K group Γ admits a faithful affine K representation making it acting on R properly discontinuously and with com- pact quotient, we say that Γ admits an affine structure. In 1977, John Milnor questioned the existence of affine structures for such groups Γ. Very recently examples have been obtained showing that, even for torsion-free, finitely gen- erated nilpotent groups N, affine structures do not always exist. It looks natural to consider affine structures as examples of polynomial structures of degree one. We introduce the concept of a canonical type polynomial structure for polycyclic-by-finite groups. Using the algebraic framework of the Seifert Fiber Space construction and a nice cohomology vanishing theorem, we prove the existence and uniqueness (up to conjugation) of canonical type polyno- mial structures for virtually finitely generated nilpotent groups. Applying this uniqueness to a result going back to Mal0cev, it follows that, for torsion-free, finitely generated nilpotent groups, each canonical polynomial structure is ex- pressed in polynomials of limited degree. The minimal degree needed for ob- taining a polynomial structure will determine the “affine defect number”. We prove that the known counterexamples to Milnor’s question have the smallest possible affine defect, i.e. affine defect number equal to one. 1. Introduction and main result Let G be a connected and simply connected solvable Lie group. Milnor ([Mil77]) conjectured that G admits a complete affinely flat structure invariant under left translations. Since then, a few papers claiming this conjecture affirmatively have been published, for example, ([Boy89], [Boy90] and [Nis90]). With this in mind, the idea of a canonical type affine representation (implicitly born in [Sch74] and [Lee83]) for (virtually) torsion-free, finitely generated nilpotent groups and the iteration problem related to this have been investigated ([DI93]). Uniqueness of these representations was studied in ([IL90]). However, today, counterexamples produced by Benoist ([Ben92]) and Burde and Grunewald ([BG93]) show that certain nilmanifolds do not admit a complete affinely flat structure. Consequently, it follows that Milnor’s conjecture is false even in the nilpotent case. Note that one can look at such a complete affinely flat structure as a “polynomial structure of degree 1”. In a situation where “polynomial structures of degree 1” fail to exist, the next best structure will be “polynomial structure of higher degree”. Received by the editors May 8, 1994. 1991 Mathematics Subject Classification. Primary 57S30, 22E40, 22E25 . Key words and phrases. Lattices in nilpotent Lie groups, complete affinely flat structures. The first author is Research Assistant of the National Fund For Scientific Research (Belgium). c 1996 American Mathematical Society 77 License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 78 KAREL DEKIMPE, PAUL IGODT, AND KYUNG BAI LEE The purpose of this paper is to prove that every torsion-free, finitely generated nilpotent group of rank K admits a faithful “canonical type” representation into the group of polynomial diffeomorphisms of RK . This result will be achieved using the algebraic framework of the so-called Seifert Fiber Space construction. As a consequence of this approach, uniqueness of such canonical representations is also proved; moreover, existence and uniqueness stay valid for groups which are virtually nilpotent. Based on the uniqueness and a well-known canonical polynomial structure found already by Mal0cev, it looks interesting to introduce the notion of affine defect number for a torsion-free f.g. nilpotent group. Groups of affine defect 0 are exactly the fundamental groups of the complete affinely flat manifolds. Groups of affine defect > 0 are those obstructing Milnor’s question. In the closing section we prove that the examples produced by Benoist, Burde and Grunewald are of affine defect 1. More precisely, the 11-dimensional nilpotent Lie groups which fail to admit a complete affinely flat structure do admit a polynomial structure of degree 2. This is the lowest possible such a structure. 2. Canonical type structures It is well known ([Seg83, Lemma 6, p. 16]) that, if Γ is a polycyclic–by–finite group, then there exists an ascending sequence (or filtration) of normal subgroups Γi (0 i c +1)ofΓ ≤ ≤ Γ :Γ0=1 Γ1 Γ2 Γc 1 Γc Γc+1 =Γ ∗ ⊂ ⊂ ⊂···⊂ − ⊂ ⊂ for which k Γi/Γi 1 = Z i for 1 i c and some ki N0 and − ∼ ≤ ≤ ∈ Γ/Γc is finite. Moreover, each Γi can be chosen to be a characteristic subgroup of Γ. Let us call such a filtration of Γ a torsion-free filtration.WeuseKto denote the Hirsch number (or rank) of Γ. Often, we will also use Ki = ki + ki+1 + + kc and ··· Kc+1 = 0. It follows that K = K1. Definition 2.1. Assume Γ is polycyclic–by–finite with a torsion-free filtration Γ . ∗ A set of generators A = a1,1,a1,2,... ,a1,k1 ,a2,1,... ,ac,kc ,α1,... ,αr of Γ will be called compatible with{ Γ iff } ∗ i 1,2,... ,c : a1,1,a1,2,... ,ai,k generates Γi. ∀ ∈{ } i It is clear at once that any torsion-free filtration Γ of Γ admits a compatible set of generators. ∗ In the sequel, frequently we will work with quotient groups of Γ. In order to avoid complicated notations, we will write the same symbols for elements in Γ, as for their coset classes in a quotient group. E.g. we say that Γ2/Γ1 is the free abelian group generated by a2,1,a2,2,... ,a2,k2 . Now we introduce quickly the basic building blocks for the representations we are interested in (see [Lee83] and [CR77]): (RK , Rk)= continuous maps λ : RK Rk , M { → } (RK )= homeomorphisms h : RK RK . H { → } License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use POLYNOMIAL STRUCTURES FOR NILPOTENT GROUPS 79 (RK , Rk) is an abelian group (for the addition of maps) and can be made into a M GL(k, R) (RK)–module, if we define ×H (g,h) 1 K k K λ = g λ h− , λ (R ,R ), g GL(k, R), h (R ). ◦ ◦ ∀ ∈M ∀ ∈ ∀ ∈H We remark that there is an embedding (RK , Rk) (GL(k, R) (RK)) , M o ×H → (Rk+K )with H (1) (λ, g, h)(x, y)=(g(x)+λh(y),h(y)), x Rk, y RK , λ (RK,Rk), g GL(k, R), h (RK). ∀ ∈ ∀ ∈ ∀ ∈M ∀ ∈ ∀ ∈H Definition 2.2. Assume Γ is polycyclic–by–finite with a torsion-free filtration Γ . K ∗ A representation ρ = ρ0 :Γ (R ) will be called of canonical type with respect to Γ iff it induces a sequence→H of representations: ∗ K ρi :Γ/Γi (R i+1 ), (1 i c) →H ≤ ≤ such that for all i the following diagram commutes: (2) k 1 Z i = Γi/Γi 1 Γ/Γi 1 Γ/Γi 1 → ∼ − → − → → j ρi 1 Bi ρi ↓ ↓ − ↓ × Ki+1 ki Ki+1 1 (R , R ) (G ) GL(ki, Z) (R ) 1 →M →Mo ×H → ×H → where Ki+1 ki Ki+1 (G ) stands for (R , R ) (GL(ki, Z) (R )), •Mo ×H M o ×H j(z):RKi+1 Rki : x z, z Zki and • → 7→ ∀ ∈ k Bi :Γ/Γi GL(ki, Z) denotes the action of Γ/Γi induced on Z i by conju- • → gation in Γ/Γi 1. − Let us establish the following results, which will be of use in a characterisation of canonical type representations. Theorem 2.3. Let Γ be any group acting on a contractible space M and containing a normal subgroup Γ0 of finite index, which acts freely and properly discontinuously on M, such that the quotient space M =Γ0 Mis a closed asphericalf manifold. \ Then, the kernel of the induced action of F =Γ/Γ0 on M is the isomorphic image of thef torsion of CΓ(Γ0) inside F . In fact, the setf of all torsion of CΓ(Γ0) forms a characteristic subgroup of Γ, and equals the kernel of the action of Γ on M. Proof. Let t be a torsion element in C (Γ ). This means that in the commutative Γ 0 f diagram 1 Γ0 Γ F 1 → → → → ↓↓ ↓ 1Inn(Γ0) Aut(Γ0) Out(Γ0) 1 →→ → → the group T generated by t maps trivially into Aut(Γ0). (The vertical maps are induced by conjugation in Γ). By [LR81, Lemma 1] the image of T in F acts trivially on M. This image is isomorphic to T ,asΓ0 is torsion-free. Conversely, if T 0 is a subgroup of F acting trivially on M,thenT0 is the isomorphic image of a subgroup T of Γ (this lifting of T 0 back to Γ can be done already if the T 0 action on M has a fixed point). Clearly, then T commutes with Γ0 so that T CΓ(Γ0). ⊂ For the last statement, note that a torsion element of Γ acts effectively on M if and only if the corresponding element of F acts effectively on M. f License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 80 KAREL DEKIMPE, PAUL IGODT, AND KYUNG BAI LEE As an immediate consequence, we find the following reformulation of the above theorem in the situation we’re interested in. Corollary 2.4. Let Γ be a polycyclic–by–finite group, with Hirsch number K.Let Nbe the Γc in a torsion-free filtration of Γ,andletF denote the set of torsion of K CΓ(N). For any properly discontinuous action of ρ :Γ R ,F is the kernel of ρ.
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